1. Field of the Invention
This invention relates to an image processing system and an image processing apparatus. More particularly, the invention relates to an image processing system and an image processing apparatus such as an image processing apparatus for performing communication between devices having different resolutions, and a printer or copier which outputs an image at a resolution different for enlargement and reduction and different from that of input resolution.
2. Description of the Related Art
Compression techniques are used in the communication of image information between devices. This is natural when one considers the quantity of information possessed by an image, and the communication of image information without compression is not conceivable at the present time.
There is a JPEG (Joint Photographic Experts Group) standard and a JBIG (Joint Bi-Level Image Experts Group) standard according to which still-picture information is coded. Though the details will not be given here, coding according to the JPEG scheme includes an orthogonal transformation based upon DCT (discrete cosine transformation) and entropy coding of coefficients obtained by quantizing the transformation coefficients. Coding according to the JBIG scheme involves a reduction method which takes into account the communication between devices having different resolutions, as well as entropy coding using arithmetic coding.
Techniques for converting resolution have long been the object of research. Such techniques are required in a case where the sizes of images are made to agree at transmission and reception when devices having different resolutions communicate with each other and in a case where an image is enlarged or reduced in size with resolution being kept the same. Various methods of converting resolution have been proposed. The method of conversion processing in these methods differs depending upon the type of image to be processed (e.g., a multivalued image having grayscale information for every pixel, a binary image binarized to pseudo-halftones, a binary image binarized simply by a fixed threshold value, a character image, etc.).
When an image is enlarged, it is necessary to interpolate a pixel having a new resolution between pixels of low resolution. To accomplish this, generally use is made of zero-order interpolation of the kind shown in FIG. 1, which arrays identical pixel values that are nearest to an interpolated point, and linear interpolation, in which a pixel value E is decided by performing the following operation based upon the distance to four points surrounding an interpolated point (where it is assumed that the pixel values of the four points are A, B, C, D), as shown in FIG. 2: EQU E=(1-i)(1-j)A+i(1-j)B+(1-i)jC+ijD
where the position of the pixel E is at distances i, j (i&lt;1, j&lt;1) from A in the horizontal and vertical directions, respectively, in a case where 1 is the distance between pixels.
Further, a method based upon sampling theory utilizing an interpolation function (a SINC function) is not used that much at present owing to the complexity of the hardware.
The communication between devices having different resolutions will be described with reference to FIG. 3. In a case where image information is transmitted between devices, the transmitting side transmits image information upon compressing and coding the information to reduce redundancy in the image. This takes place in a compression unit 1001. The receiving side decodes and decompresses the received code by using a decompression unit 1002 and then obtains an image upon effecting a conversion to the resolution on the receiving side using a resolution converter 1003. With the exception of compression or transmission applied to a binary image according to the JBIG scheme, the transmission of a multiple-tone image generally is performed in the manner shown in FIG. 3. In a case where image information is created in a host computer and outputted to a printer, an image made to conform to the resolution of the printer is created by the host computer using a resolution converter 1004, the image is compressed by a compression unit 1005 and is then transmitted. Upon receiving the image, the printer decompresses it using a decompression unit 1006 and outputs the decompressed image.
In a case where the image compressed includes a multiple-tone natural image and a two-tone character or line drawing mixed in a single image, the simplest compression method would be to quantize orthogonal transformation coefficients using a predetermined quantization table after the orthogonal transformation is performed, as in the JPEG base-line system. If priority is to be given to image quality, the method used would be to change over the quantization conditions adaptively depending upon the local properties. For example, with regard to a character or line drawing, the spatial frequency components contain many high-frequency components and therefore control would be performed to establish quantization conditions according to which coarse quantization is not executed in the high-frequency region.
A scheme of the kind shown in FIG. 5 has also been proposed. Here the transmitting side first reduces high-frequency components in the image by a pre-filter 1010, executes an orthogonal transformation in an orthogonal transformation unit 1011 and transmits the image upon quantizing it in a quantizer 1012. The receiving side performs a reverse quantization in a reverse quantizer 1013, applies an inverse orthogonal transformation in an inverse orthogonal transformation unit 1014 and then decodes the image upon applying processing such as edge emphasis using a post-filter 1015. A hybrid method has also been proposed, according to which a natural image portion and character or line-drawing portion are separated from an image. The natural image is irreversibly compressed utilizing an orthogonal transformation and quantization, and the character or line drawing is reversibly compressed by run-length coding or MMR. In a case where image information is communicated between devices having different resolutions using these methods, an image made to conform to the resolution on the receiving side is obtained by subjecting the image information to zero-order interpolation or linear interpolation on the receiving side or transmitting side.
However, the following problems arise in the examples of the prior art mentioned above:
When quantization is performed under the same conditions irrespective of the characteristics of the image using the JPEG base-line system, a large quantization error in the high-frequency region at the character or line-drawing portion is produced and ringing noise referred to as "mosquito noise" becomes visible.
Further, with the method of changing over quantization conditions, the compression efficiency of characters of line drawings deteriorates markedly and the total sum of amount of code fluctuated widely depending upon the proportion of the character or line drawing.
When the method using the pre-filter and post-filter and the resolution transformation method shown in FIG. 3 are combined, the conversion of resolution is carried out after post-filtering. As a consequence, noise is increased and a deterioration in quality tends to be sensed visually. When the same filters are combined with the form of resolution conversion shown in FIG. 4, the amount of information prior to compression becomes very large when the resolution of a printer, for example, is high, and the range covered by the effects of filtering becomes relatively narrow.
The hybrid method must use different compression schemes for natural image portions and character or line-drawing portions. This leads to higher costs and an increase in hardware.
Furthermore, a problem is encountered in the resolution conversion itself. Specifically, though the method illustrated in FIG. 1 is advantageous in terms of its simple configuration, pixel values are decided for every block enlarged in a case where the method is applied to natural image. As a result, the blocks become conspicuous. In a case where the method is applied to a character, line drawing or computer graphics, the same pixel values are rendered continuous for each block enlarged. The result is an image of conspicuous roughness or "jaggies", especially along diagonal lines. On the other hand, the method of FIG. 2 is used generally to enlarge a natural image. Though the resulting image has an averaged and smoothed image quality, the occurrence of jaggies in characters or line drawings is unavoidable and the edges of images become blurred.